The present invention relates to a wheel-support rolling bearing unit, which is used for rotatably supporting vehicle wheels on suspension devices, and also relates to a method of manufacturing the same.
The vehicle wheels are supported on the suspension devices by the wheel-support rolling bearing unit. FIG. 4 shows a conventional wheel-support rolling bearing unit widely used. The conventional wheel-support rolling bearing unit 1 includes a hub 2, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5. A first flange 6 for supporting the wheel is formed at the outer end of the outer peripheral surface of the hub 2 (the word “outer side or outside” means the outer side of the vehicle body as viewed in the widthwise direction in a state that the unit is assembled into the motor vehicle, and the left side in those figures except FIG. 8. The word “inner side or inside” means locations closer to the center of the vehicle body as viewed in the widthwise direction, and the right side in those figures except FIG. 8.). A first inner raceway 7 is formed in the outer surface of a middle portion of the hub 2. A stepped part 8 having the reduced outside diameter is formed at the inner end thereof.
The inner ring 3 having a second inner raceway 9 formed therein is fit to the stepped part 8. A male threaded portion 10 is formed in the inner end of the hub 2. The top end (the right end in FIG. 4) of the male threaded portion 10 is extended inward beyond the inner end face of the inner ring 3. The inner ring 3 is held between the nut 11 screwed to the male threaded portion 10 and a step surface 12 of the stepped part 8, whereby the inner ring 3 is firmly coupled to a predetermined position on the hub 2. An engaging recess 13 is formed in the outer peripheral surface of the top end of the male threaded portion 10. The nut 11 is fastened by a predetermined torque, and then a part of the nut 11, which is aligned with the engaging recess 13, is radially and inwardly caulked, thereby preventing the loosening of the nut 11.
A first outer raceway 14 and a second outer raceway 15 are formed in the inner peripheral surface of the outer ring 4. The first outer raceway 15 is confronted with the first inner raceway 7. The second outer raceway 15 is confronted with a second inner raceway 9. A plurality of rolling elements 5 are located between the first inner raceway 7 and the first outer raceway 14. A plurality of rolling elements 5 are located between the second inner raceway 9 and the second outer raceway 15. In the illustrated instance, the rolling elements 5 are balls; however, in the case of the wheel-support rolling bearing unit for motor vehicles which is heavy, those may be tapered rollers.
To assemble the wheel-support rolling bearing unit 1 into the motor vehicle, the outer ring 4 is fixed to a suspension device with a second flange 16 formed on the outer peripheral surface of the outer ring 4, and the wheel is fixed to the first flange 6. As a result, the wheel is rotatably supported on the suspension device.
In Japanese Patent Unexamined Publication No. Hei.11-129703, there is described a wheel-support rolling bearing unit 1a as shown in FIGS. 5 to 7. The wheel-support rolling bearing unit 1a as a second conventional technique includes a hub 2a, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5. A first flange 6 for supporting the wheel is formed at a portion of the hub 2a which is closer to the outer end of the outer peripheral surface thereof. A first inner raceway 7 is formed in the outer surface of a middle portion of the hub 2a. A stepped part 8 having the reduced outside diameter is formed at the inner end thereof.
A cylindrical portion 18, which is used for forming a caulking portion 17 to fix the inner ring 3, is formed in the inner end of the hub 2a. A thickness of the cylindrical portion 18 becomes small toward its top end (the right end in FIG. 7) before the cylindrical portion 18 shown in FIG. 7 is expanded, for caulking, outward in the diameter directions. Accordingly, a tapered hole 19 is formed in the inner end face of the hub 2a. The inside diameter of the tapered hole becomes small toward a recess.
To expand, for caulking, the top end of the cylindrical portion 18 in order to fasten the inner ring 3 to the inner end of the hub 2a, a force piston 20 is forcibly pressed against the top end of the cylindrical portion 18 in a state that the hub 2a is fixed so as not to shift in the axial direction, as shown in FIG. 6. An outward curved portion 21, shaped like a truncated cone, which may be thrust into the cylindrical portion 18, is formed in the central portion of the end surface (left end surface in FIG. 6) of the force piston 20. An inward curved portion or recess 22 is formed surrounding the outward curved portion 21. A cross section of the recess 22 is configured to have a complex curved surface whose radius of curvature becomes small toward the outside so that a cross section of the caulking portion 17, which is formed by plastically forming the top end of the cylindrical portion 18 by the recess 22, gradually reduces in size from the base end to the top end, and abruptly reduces at the top end.
By pressing the force piston 20 having the outward curved portion 13 and the recess 22, which are shaped and have dimensions as mentioned above, against the top end of the cylindrical portion 18, the top end of the cylindrical portion 18 is forcibly expanded outward in the diameter directions for caulking, to thereby form a caulking portion 17. The inner ring 3 is firmly held between the caulking portion 17 and a step surface 12 of the stepped part 8 formed at the inner end of the hub 2a, whereby the inner ring 3 is fastened to the hub 2a. 
For a work to form the caulking portion 17 by plastically deforming (expanding for caulking) the cylindrical portion 18, it is preferable to use a swing press 23 as shown in FIG. 8. The swing press 23 is made up of a force piston 20, a holding tool 24 and a holder 25. In forming the caulking portion 17 by expanding for caulking the cylindrical portion 18, the force piston 20 is displaced in a swing manner while pushing upward the hub 2a with the aid of the holder 25. In a state that the center axes of the force piston 20 and the hub 2a are inclined at an angle θ with respect to each other, the force piston 20 is turned about the center axis of the hub 2a, while being in contact with each other. In forming the caulking portion 17 by the swing press as just-mentioned, a part of the circumference of the force piston 20 is pressed against the cylindrical portion 18, so that the caulking expanding work of the caulking portion 17 continuously progresses in part in the circumferential direction. For this reason, when comparing with the forming of the caulking portion 17 by ordinary forging process, a load acting on the cylindrical portion 18 may be reduced during the forging process. The holding tool 24 prevents the inner ring 3 and the hub 2a from moving in the radial direction during the caulking expanding work of the caulking portion 17 by the force piston 20.
In the conventional wheel-support rolling bearing unit thus constructed, it is difficult to apply a pre-load to the rolling elements 5 and to render normal a fitting state of the inner ring 3 to the hub 2a. In recent years, a measure in which the pre-load is applied to the rolling elements 5 has been employed in order to increase a rigidity of the wheel-support rolling bearing unit. The related component parts are dimensioned so that the pre-loading has an appropriate value in a state that the outer end face 28 of the inner ring 3 is in contact with the step surface 12 formed on the outer peripheral surface of the hub 2a. 
In a state that the outer end face 28 of the inner ring 3 is brought into contact with the step surface 12 to pre-loading the rolling elements 5, a thrust load acts on the inner ring 3 based on the pre-loading, in the direction causing the inner ring 3 to move apart from the step surface 12. In the case of the conventional structure, the inner ring 3 is axially displaced by such a thrust load before the caulking portion 17 is formed at the inner end of the hub 2a, and as a result, the outer end face 28 is separated from the step surface 12.
If the work to form the caulking portion 17 starts from a state that the outer end face 28 of the inner ring 3 is separated from the step surface 12, the inner ring 3 is pressed against the step surface 12, while being scraped off. Specifically, with the formation of the caulking portion 17, the inner ring 3 is pushed onto the stepped part 8. At this time, the direction of the force pressing the inner ring 3 is slanted with respect to the directions of the axes of the inner ring 3 and the hub 2a. Accordingly, the circumferential edge of the opening of the outer end of the inner ring 3 is pushed onto the stepped part 8 while forcibly rubbing against the outer peripheral surface of the stepped part 8 (biting into the outer peripheral surface of the stepped part 8).
As a result, a small depression is formed in the outer peripheral surface of the stepped part 8. This possibly leads to degradation of the roundness deviation of the second inner raceway 9, which is formed in the outer peripheral surface of the inner ring 3, and an inclination (small as it is) of the center axis of the inner ring 3 with respect to the center axis of the hub 2a. The degradation of the circularity deviation and the center axis inclination are both undesirable since those will cause the vibration during the operation of the wheel-support rolling bearing unit to increase and the unit performances to deteriorate.